Officers arrested a 22-year-old, who was seen acting suspiciously, after a short chase near a Morrison's supermarket in the city.

Christina, a pupil at Leasowes High School in Halesowen, was an avid user of the social networking site Twitter and on February 8 the @christinaedkin account included a post that read: "This man is worrying me on the bus keeps getting up and walking up and down stairs, and sitting in different place :I"

West Midlands Police said they would be looking at her social media activity, but that "at this stage there is nothing to suggest she was being targeted".

Officers said that there was "nothing immediately" to suggest that Christina knew her attacker and that he had already been on the bus when she got on board.

The head teacher at Christina's school, Neil Shaw, said: "Christina was a bright and popular student much loved by staff and students alike. We are deeply saddened to hear this tragic news and our thoughts and hearts go out to her family and everyone who knew her.

"The school is working closely with the police and a team of counsellors to provide support to our pupils and staff."

Christina had recently taken part in Twitter "meet ups", where social media users get together to socialise.

Image Caption:Christina was stabbed on the crowded number 9 service

Joe James, 18, from Bristol got to know Christina through these events and told Sky News: "I'm still not believing it. I just can't understand - why her?

"She had everything, she was very popular and generous - she was the kind of girl to help an old lady across the road."

A tribute page has been set up on Facebook for "beautiful angel" Christina, with users calling her death "a waste of a beautiful young life" and saying "justice will prevail".

Detective Superintendent Richard Baker said: "This is a tragic case and the victim's family are devastated.

"We are still anxious to hear from anyone with information or who may have witnessed the incident this morning to speak to us via the dedicated hotline 0800 096 0095.

"There will be a high police presence across the city this afternoon following this morning's tragedy particularly on buses and in schools in the area to provide reassurance to local people."

He said that he thought a number of people travelling on the bus at the time of the attack on Hagley Road had yet to come forward and that officers were trawling through CCTV footage for evidence.

Image Caption:Paramedics were called but nothing could be done to save the teenager

National Express West Midlands said the vehicle was heading out of the city when the stabbing happened.

A spokesman said: "We can confirm that a tragic incident occurred on a number 9 service this morning.

"Our thoughts and condolences are with the friends and family of the person involved and we will offer all possible support to the police in their investigation of this matter."

A West Midlands Ambulance Service spokesman said they had been called to the scene shortly after 7.35am.